Process for the electrolytic deposition of aluminium



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United States Patent PROCESS FOR THE ELECTROLYTIC DEPOSITION OF ALUMINIUM Karl Ziegler and Herbert Lehmkuhl, Mulheim an der Ruhr, Germany; said Lehmkuhl assignor to said Ziegler No Drawing. Application July 11, 1955 Serial No. 521,424

Claims priority, application Germany July 28, 1954 23 Claims. (Cl. 204-14) This invention relates to a process for the electrolytic deposition of aluminium.

The problem of electrolytically depositing aluminium under gentle temperature conditions and with a low current consumption has not so far been solved satisfactorily.

The production of aluminium on a technical scale exclusively makes use of melt electrolysis at SOD-900 C. in an electrolyte consisting mainly of molten cryolite. The current consumption is about 24 kilowatt hours per kilogram of aluminium.

The same process is used in principle in the electrolytic refining treatment in the so-called three layer process. The current consumption is somewhat lower, but nevertheless is still 20 kw.-h./ kg.

A series of electrolytes have been proposed for electrolysis at relatively low temperatures, but none of these, however, have actually been used in practice.

In addition to certain complex compounds of aluminium chloride with organic bases, such as pyridine, the literature has referred to certain purely inorganic electrolytes, such as, for example, sodium-aluminium chloride, in the melt or also more recently a complex aluminium compound which is formed from lithium hydride and aluminium chloride in ethereal solution (D. E. Couch and A. Brenner, Journal Electrochemical Society 99 (1952), 234).

There has not so far been any disclosure concerning the use of such electrolytes on a technical scale.

It has now been found that under certain conditions, excellent electrolytes for the deposition of aluminium under gentle temperature conditions can be synthesised from true organic aluminium compounds i. e. from such compounds which contain at least one carbon atom directly bonded to aluminium.

Aluminium trialkyls, aluminium dialkyl hydrides AlR H, aluminium alkyl halides, such as AlR Cl, AlRCland similar substances are all non-conductors of electric current. However, they can be transformed into electrolytes by converting them into complex compounds, for example sodium ethyl with aluminium triethyl forms sodium-aluminium tetraethyl NaAl(C H In molten form, this substance has very good conductivity. This has already been shown by F. Hein (Zeitschrift fiir anorganische Chemie, 141, 161-226 (1924)), who obtained an oily product from sodium ethyl and aluminium triethyl and investigated it concerning its conductivity; it is true that he designated this product as being a solution of sodium ethyl in aluminium triethyl, but, as is known for certain at the present time, this product was the complex compound just referred to. F. Hein did not indicate which element is deposited on the cathode in the electrolysis. If this is investigated, it is found that mainly sodium is formed on the cathode. In addition, some aluminium may also be deposited, and spongy mixtures of sodium and aluminium are frequently observed to form at the cathode. The ratio between the amounts of sodium and aluminium which are formed is dependent on the current density. It is mainly aluminium which appears with very low current densities, but aluminium is never exclusively obtained with current densities which can be used on a technical scale. This electrolyte therefore cannot be used for the deposition of aluminium. Similar remarks apply as regards a number of other such organic complex compounds, for example KAI(C2H5)2CI3 and NaAl(C I-I F.

Excellent electrolytes of high specific conductivity for the deposition of metallic aluminium are, however, always obtained if electrolytes are used which also contain an excess of the organic aluminium compounds in addition to such complex compounds. Such electrolytes or electrolyte mixtures can not be produced as required, perhaps by combining aluminium triethyl with an one of the aforementioned complex compounds, in every case since frequency two basically possible components of such a mixture are actually not homogeneously miscible with one another. For example, a homogeneous mixture or solution can not be produced from the aforementioned sodium-aluminium tetraethyl and aluminium triethyl, since the two substances are in practice completely insoluble in one another. On the other hand, in all the combinations in which a sufiicient mutual miscibility or solubility exists, it is also possible to observe that pure aluminium of excellent quality is deposited during the electrolysis.

Electrolytes which have proved to be particularly suitable are homogeneous melts of true organic aluminium compounds of the general formula AlR(R') in which R represents an alkyl radical and R represents an alkyl radical or a hydrogen or halogen atom, together with complex compounds thereof with alkali metal compounds of the general formula MeR, wherein Me represents an alkali metal, or quaternary ammonium compounds.

Alkali metal compounds which are particularly suitable for forming the complex are alkali metal alkyls, hydrides and halides, and also tetraethyl ammonium halides, triethyl-n-butyl-ammonium halides, ethyl-tri-nbutylammonium halides, dodecyl-tri-methyl-ammonium halides, such as the chloride, and also quaternary pyridinium, quinolinium and isoquinolinium salts, such as pyridinium methiodide.

The electrolytes can be homogeneously melting compounds of true organic compounds of the general formula AlR(R) in which R represents an alkyl radical and R represents an alkyl radical or a hydrogen or halogen atom, and their complex compounds with alkali metal compounds of the general formula MeR, wherein Me represents an alkali metal, or with quaternary ammonium compounds.

As homogeneous melts, it is also possible to use mixtures of true organic compounds of the general formula AlR(R in which R which represents an alkyl radical and R represents an alkyl radical or a hydrogen or halogen atom, and their complex compounds with alkali metal compounds of the general formula MeR, wherein Me represents an alkali metal, or with quaternary ammonium compounds, or homogeneously melting solutions of these substances.

For example, a suitable electrolyte is obtained by dissolving 1 mol of sodium-aluminium diisobutyl dihydride- Na(Al= (iso=C H H in aluminium triethyl. The basic complex compound is obtained very easily by adding sodium hydride to diisobutyl aluminium hydride. Conversely, it is obviously also possible to obtain the same electrolyte from sodium-aluminium triethyl hydride and diisobutyl aluminumhydride, or even more simply by dissolving sodium hydride in a mixture of aluminium triethyl and diisobutyl aluminium hydride.

Organic aluminium complex compounds containing fluorine have proved to be particularly suitable for com- Patented Aug. 26, 1958 bining'to give electrolytes of the type referred to. For example, the sodium-aluminium triethyl fluoride NaAl Cal-I5) F with-2 mols-'of aluminium triethyl forms a novel and welldefined :second complex compound with the composition NaF.2Al(C H this compound is described in a prior application of the applicants and melts at a ve'r-ylowtemperature. The specific conductivity of the melt is as follows:

3.3 X "ohmc):n.- at 24 C. and

15.0 X 10- ohm 'cm.- at 80 C. and

and yields byelectrolysis a dense firmly adhering aluminium of excellent quality. Similarly, an electrolyte can be produced by dissolving sodium fluoride in a mixture of'aluminium trimethyl and aluminium triethyl. In this case, it is again a question of the formation of -a complex compound of 1 mol of sodium fluoride with -2 mols of aluminium trialkyls (which are here differentfrom one another).

In the electrolyte mixtures to be described, it is at present not possible to indicate which complex separate compounds are actually present in the liquid electrolyte.

An example of an excellent electrolyte is one which has exactly or substantially the following composition in themelt:

Having regard to the course of the following reactions:

it 'can also be written as:

as can easily-be seen by its decomposition into This electrolyte can be produced by melting together sodium fluoride, aluminium triethyl and aluminium diethyl "fluoride. Another excellent electrolyte of this group is formed if finely powdered dry cryolite is boiled with more than 6 mols of aluminium triethyl. The cryolite enters into solution. Two layers are formed, the upper layer consisting of aluminium triethyl, which can easily be separated while the lower layer has the composition:

Similar electrolytes are set out in the 'followin'g'table. The compositions set out therein do not have to'be exactly fulfilled, but only approximately. The formulae in the table are written in such a way that it can be seen from what basic substances the electrolytes are formed. If it were desired for all formulae to be correctly expressed in the chemical sense, it would be necessary in every case for 1 mol of the alkali halide, alkyl or hydride present in the electrolyte to be combined with 1 mol of the organic aluminium compound to form a complex of the Me(AlR X) type. It will be seen from the formulae of he electrolytes indicated below that in all cases 1 mol of the organic aluminium compound is available for each mol of alkali halide, alkyl or'hydride, thus conforming to th'eprinciple of the process of the invention:

KF ZAKCzHs): NaH Al(-C|Ht): AK a Jh NaF Axum), AI(CH;)| NaF 2A1(C2H6)a N8F ZAKCtHo);

By selecting suitable quaternary ammonium salts, it is possible to produce any desired solutions of the quaternary salts in aluminium triethyl. In such electrolyte combinations, liquid phases of the predetermined composition of one salt molecule and two aluminium trialkyl molecules often do not form. It is then possible with a very wide range of variation to produce solutions which contain only a small amount of electrolyte and a large amount-of aluminium triethyl.

Many of the aforementioned electrolytes can be mixed as desired or to a limited degree with solvents. The only solvents which can be used are those which do not decompose the organic aluminium compounds, for example bydrocarbons, especially those of aromatic nature. Ethers and tertiary amines, such as tetrahydrofurane, dime'thyl aniline, dioxane and pyridine can also be used.

It is not recommended that the dilution of the electroly'tes by such solvents should be carried too far, since then the specific conductivity is lowered and consequently a higher consumption energy is required for deposition of 'the aluminium. Such dilution, may, however, be advisable for protecting the electrolytes, which are very than 'C. and up to a maximum of 200 C. have frequently proved to be desirable, even if not necessary. Hig'h boiling aromatic substances, such as methyl naphthalene, diphenyl ether and tetraline, are suitable as diluen'ts in such cases.

If the operation is carried out Without diluents, which is frequently advantageous, the heated melts of the electrolytes can be very easily protected against the action of air by covering them with a small amount of paraffin oil. They are not all miscible with paraflin oil. However, even while observing this precautionary measure, it is advisable to carry out the electrolysis processes in closed vessels and to fill the free space above the oil with an inert protective gas, such as nitrogen.

It is further to be mentioned that obviously several of the aforementioned electrolytes can also be mixed with one another in suitable manner; for example, certain amounts of sodium aluminum tetraethyl can also be dissolved in the NaF.(Al(C H electrolyte without impairing the usefulness of the electrolyte. The excess of aluminum triethyl which even then is still present, is sufficient. The electrolytes enumerated in detail and by way of example contain methyl, ethyl and l-butyl as organic radicals bonded to aluminium. It is obvious that a large number of similarly usable electrolytes can also be produced by using other alkyl radicals and'also by using those having a higher number of carbon atoms. This may offer certain advantages, since the sensitivity to air and especially the capacity for spontaneous ignition decreases with the large alkyl radicals. However, it will be understood that as the organic portion of the electrolyte systems becomes greater, the specific conductivity values must drop, since 'a'ltogether'there are'fewer ions per unit of volume. Consequently, the change-over to such higher alkyl compounds usually does not offer any particular advantages. Nevertheless, exceptions to the rule are obtained when the costs of the electric energy to be used are not important, as for example, when producing thin aluminium covering layers on wires.

The novel electrolytes can be used with advantage in all cases where it is important that a very pure aluminium should be deposited, i. e. they can be usedfor the production of aluminium coatings, possible on other metals such as copper, or also on a foundation material of less pure aluminium, or also for refining aluminium. In all cases, the electrolysis is preferably carried out using aluminium anodes.

In this cases, practically no losses of electrolyte occur, providing that the current density remains below about 2 am./dm. On the other hand, if the operation is carried out with anodes of copper, iron or platinum, for example, gas is evolved .at the anodes. When the electrolytes contain the ethyl radical, the gases consist of mixtures of ethane and ethylene, and in such a case the aluminium trialkyl added in excess to the electrolyte gradually becomes exhausted during the electrolysis. Obviously, it is possible to compensate for this by adding the necessary amounts of the organic aluminium compound from time to time. When using aluminium anodes, a similar evolution of gas only becomes apparent at a high current density. Otherwise, normally the amount of aluminium which dissolves at the anode is that which has been deposited at the cathode.

If an ordinary foundry aluminium is used as anode, the impurities of the aluminium remain undissolved in the form of a black sludge. The sludge initially remains loosely clinging to the anode, but usually becomes detached during the electrolysis and falls to the bottom of the cell, but frequently also remains suspended in the electrolyte to some extent, with the possible result that particles of the anode sludge adhere to the cathode coatings and impair the deposition thereon.

This difiiculty can be overcome in a very simple manner by surrounding the anodes by a tightly fitting bag of an ordinary cotton fabric, whereby the anode sludge is completely held back. Such protective devices for the anode surprisingly have an extraordinarily long effective life, despite the relatively high temperatures of the electrolyte, which are usually 60 to 150 C. and even up to 200 C.

The voltage during the electrolysis between terminals consisting of parallel plates of equal size can easily be maintained between 0.3 and 1 volt, when the spacing of the plates is about 3 cm. Under these circumstances, the energy consumption per kg. of deposited aluminium is between 0.9 and 3 kw.-h. at 150 C.

The aluminium coatings are initially dense and compact, but as the thickness of the deposited layer increases, they become gradually granular and blistered. However, the aluminium can be deposited up to a considerable thickness without the cells being damaged by bridge formation. Within the range of layer thicknesses which are used for surface treatment, the coating is completely dense and smooth and adheres firmly, provided that the surface of the foundation material has been thoroughly cleaned. A spectroscopic examination of the deposited aluminium shows that it is at least 99.999% pure, even when ordinary foundry aluminium has been used as anode. When using quaternary ammonium compounds for the formation of the complex compounds, a large number of dilferent electrolytes are available, by which the properties of the aluminium deposit, especially the hardness, the lustre and the grain structure can be influenced as required. For the production of aluminium coatings on metal surfaces, it may be advisable to use anodes of ultrapure aluminium. In this case, it is unnecessary for the electrolyte to be protected in the manner described above against contamination by the anode sludge. The purity of the aluminium produced by the process of the invention is apparent from the following experiments as well as from the spectroscopic examination:

An aluminium coating produced on a polished steel sheet as support was removed as a foil and cut into pieces about 2 cm. square. 5 cc. of 20% hydrochloric acid was poured over one such piece. For comparison purposes, the same amount of an aluminium sheet with 99.8% aluminium was subjected to the same test. In the comparison experiment, the temperature in the acid rose within 12 minutes to 72 C. and the aluminium had then completely dissolved with violet evolution of hydrogen. The electrolytically deposited aluminium did not produce any rise in temperature and remained substantially undissolved in the hydrochloric acid for some hours.

In another similar pair of experiments, the aluminium obtained according to the process of the present invention was compared with the best quality refined ultra-pure aluminium (99.99% It was not possible to detect a rise in temperature in either of the two cases, but the 99.99% aluminium had completely dissolved after 12 hours. The aluminium obtained by a process of the invention merely showed a decrease in weight by about /3.

Copper plates or copper wires which have been provided by the process of the invention with an aluminium coating a few microns thick can be kept for hours in nitric acid without it being possible to detect any corrosion of the copper by the nitric acid.

The particularly high purity of the aluminium obtained is obviously the result of the fact that all electrolytes had been prepared from distillable and also distilled organic aluminium compounds. By this means it is clearly possible for the normal impurities of the aluminium to be separated with very great effectiveness in a very simple manner.

The following examples further illustrate the invention:

Example 1 42 g. of sodium fluoride and 260 g. of aluminium triethyl are brought together under a nitrogen atmosphere and heated while stirring to l00-120 C. The solid salt dissolves and two liquid layers are obtained. The upper layer consists of practically pure triethyl and the bottom layer is an excellent electrolyte for the electrolytic deposition of aluminium and has exactly the composition NaF.2Al(C H Example 2 melt electrolysis of this reaction product, a firmly adhering aluminium deposit is obtained on the cathode.

Example 3 58 g. of potassium fluoride are dissolved in 198 g. of aluminium triisobutyl H CH3 Al(CHr-\ )2 at about 100 C. in a nitrogen atmosphere. 85 g. of aluminium trimethyl are added to this reaction mixture while stirring and heating to -90 C. Two liquid layers are formed. The upper layer is. practically non-conductive to electric current, andthe bottom layer has the composition KF;Al(i-C H .Al(CH and is quite suitable for the electrolytic deposition of aluminium.

Example 4 24 g, of sodium hydride, 142 g. of aluminium diisobutyl hydride H CH3 M r-C )2 and 114 g. of aluminium triethyl are combined in a nitrogen atmosphere and heated while stirring to 110-120". The solid sodium hydride dissolves. The liquid reaction mixture which is formed has the composition NaH.HAl i-C4H9) C2H5 3 Example 5 mol (=55 g.) of potassium chloride and 1 mol (=120.5 g.) of aluminium diethyl monochloride.

ClAl (C I-I 2 are brought together under nitrogen. If the reaction mixture is heated to 100 C. the potassium chloride dissolves. A homogeneous liquid is formed which solidifies on cooling. During the melt electrolysis of this reaction product, a solid aluminium precipitate is deposited on the cathode.

Example 6 Pyridinium methiodide is brought together with aluminium triethyl, as described in Example 6. In this case also,

a yellowish to brownish liquid bottom layer is formedwhich is not miscible with aluminium triethyl, the said layer having the composition pyridinium methiodide.2 aluminium triethyl.

Example 8 Dodecyl-trimethyl-ammonium chloride is dissolved in liquid aluminium tri-n-butyl in such an amount that the molar ratio (mols) between the quaternary salt and the aluminum tri-n-butyl is greater than 1. Electrolytes of any desired composition are obtained (the quaternary salt being soluble in aluminium tri-n-butyl in all proportions) and these electrolytes deposit a good and firmly adhering precipitate on the cathode during the electrolysis.

What we claim is:

l. A process for the electrolytic deposition of aluminum upon an electrically conductive surface which comprises passing an electric current between an anode and said electrically conductive surface as a cathode through an electrolyte which consists essentially of a homogeneous melt of a true organic compound of aluminum of the general formula AIR(R') in which R is an alkyl group and R is a substituent selected from the group consisting of alkyl, hydrogen and halogen, in combination with a complex compound of such aluminum compound AlR(R) with a compound of the formula MeR in which Me is selected from the group consisting of alkali metals and quaternary ammonium radicals and R has the same significance as above, said complex compound being of the formula MeRAlR(R) in which Me, R and R have the same significance as above, the quantity of said organic compound of aluminium of the formula AlR(R) in said homogeneous melt being in excess of that contained in said complex compound of the formula MeR'AlR(R') 2. The process of claim 1 in which said electrolyte essentially consists of a compound of said aluminum compound AlR(R') with said complex compound.

3. The process of claim 1 in which said electrolyte essentially consists' of a mixture of said aluminum compound AlR(R') and said complex compound.

4. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AIR(R') and said complex compound.

5. The process of claim 4 in which an organic solvent which does not decompose the organic aluminum compound is the solvent for said solution.

6. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AIR(R') and said complex compound in which a hydrocarbon solvent is the solvent for said solution.

7. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AlR(R') and said complex compound in which an aromatic hydrocarbon solvent is the solvent for said solution.

8. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AlR(R) and said complex compound in which an ether is the solvent for said solution.

9. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AlR(R')- and said complex compound in which tetrahydrofurane is the solvent for said solution.

10. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AlR(R) and said complex compound in which dioxane is the solvent for said solution.

11. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AlR(R') and said complex compound in which a tertiary amine is the solvent for said solution.

12. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AlR(R') and said complex compound in which dimethyl aniline is the solvent for said solution.

13. The process of claim 1 in which said electrolyte essentially consists of a solution of said aluminum compound AlR(R') and said complex compound in which pyridine is the solvent for said solution.

14. The process of claim 1 in which said complex compound contains fluorine.

15'. The process of claim 1 in which said electrolyte essentially consists of a compound of the formula NaF.2A1(C H 16. The process of claim 1 in which said electrolyte essentially consists of a compound of the formula NaF.Al(CI-I .Al(C H 17. The process of claim 1 in which said electrolyte is covered with parafiin oil.

18. The proces of claim 1 in which said electrolyte is maintained under an inert gas atmosphere.

19. The process of claim 1 in which said electrolyte is maintained at a temperature between 60 and 200 C.

20. The process of claim 1 in which said electrolyte is maintained at a temperature between and C.

21. The process of claim 1 in which said anode is an aluminum anode.

22. The process of claim 1 in which said electrically conductive surface is a surface of another metal.

23. The process of claim 1 in which said cathode is of shaped metal and the aluminum is deposited thereon as a coating.

(References on following page) References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS Chimica E. Industria, K. Ziegler, 1952, vol. 34, pp.

2,321,367 Diggin June 8, 1943 520427- tudies in the Electrode osition of Metals Bulletin No. 2,446,34 t 1. A 3, 1948 Q 2,728,713 2:32 27, 1955 5 206, pubhshed by Umverslty of Illmols, pp. 10 and 11.

Z. Anorg. Chernie, F. Hein, 141, 161-226, 1924. 

1. A PROCESS FOR THE ELECTROLYTIC DEPOSITING OF ALUMINUM UPON AN ELECTRICALLY CONDUCTIVE SURFACE WHICH COMPRISES PASSING AN ELECTRIC CURRENT BETWEEN AN ANODE AND SAID ELECTRICALLY CONDUCTIVE SURFACE AS A CATHODE THROUGH AN ELECTROLYTE WHICH CONSISTS ESSENTIALLY OF A HOMOGENEOUS MELT OF A TRUE ORGANIC COMPOUND OF ALUMINUM OF THE GENERAL FORMULA AIR(R'')2 IN WHICH R IS AN ALKYL GROUP AND R'' IS A SUBSTITUENT SELECTED FROM THE GROUP CONSISTING OF ALKYL, HYDROGEN AND HALOGEN, IN COMBINATION WITH A COMPLEX COMPOUND OF SUCH ALUMINUM COMPOUND AIR(R'')2 WITH A COMPOUND OF THE FORMULA MER'' IN WHICH ME IS SELECTED FROM THE GROUP CONSISTING OF ALKALI METALS AND QUATERNARY AMMONIUM RADICALS AND R'' HAS THE SAME SIGNIFICANCE AS ABOVE, SAID COMPLEX COMPOUND BEING OF THE FORMULA MER''AIR(R''2) IN WHICH ME R AND R'' HAVE THE SAME SIGNIFICANCE AS ABOVE, THE QUANTITY OF SAID ORGNAIC COMPOUND OF ALUMINUM OF THE FORMULA AIR(R'')2 IN SAID HOMOGENEOUS MELT BEING IN EXCESS OF THAT CONTAINED IN SAID COMPLEX COMPOUND OF THE FORMULA MER''AIR(R'')2. 